Optical pickup and information equipment

1. An optical pickup apparatus comprising: an irradiating device for irradiating a laser beam to perform at least one of a recording operation and a reproducing operation on a recording medium; a first light focusing device which is disposed on an optical path of the laser beam between said irradiating device and the recording medium and which focuses the laser beam on the recording medium; a collimator device which is disposed on the optical path between said irradiating device and said first light focusing device and which can be displaced along the optical path of the laser beam; a splitting device which is disposed on the optical path between said collimator device and said first light focusing device, which reflects one portion of the laser beam, and which transmits therethrough another portion of the laser beam; a light receiving device for receiving the one portion of the laser beam reflected by said splitting device; and a second light focusing device for focusing, on said light receiving device, the one portion of the laser beam reflected by said splitting device, at least one of said light receiving device and said first light focusing device being disposed at a position at which rate of change in light density of the laser beam on said first light focusing device before and after displacement of said collimator device is substantially the same as rate of change in light density of the laser beam on said light receiving device before and after the displacement of said collimator device.

2. The optical pickup apparatus according to claim 1 , wherein each of a focal length of said second light focusing device and a distance between said second light focusing device and said light receiving device is set such that the rate of change in light density of the laser beam on said first light focusing device before and after the displacement of said collimator device is substantially the same as the rate of change in light density of the laser beam on the first light focusing device before and after the displacement of said collimator device, the rate of change in light density of the laser beam on said first light focusing device being determined in accordance with the focal length of said second light focusing device and the distance between said second light focusing device and said light receiving device.

3. The optical pickup apparatus according to claim 1 , wherein at least one of said light receiving device and said first light focusing device is disposed at a position at which aspects of the change in light density of the laser beam on said light receiving device and said first light focusing device before and after the displacement of said collimator device are in a linear relation.

4. The optical pickup apparatus according to claim 1 , wherein a spot diameter of the laser beam irradiated on said light receiving device is greater than said light receiving device.

5. The optical pickup apparatus according to claim 1 , wherein if light density on said first light focusing device increases as said collimator device is displaced close to said light receiving device, said tight receiving device is disposed on a closer side, viewed from said second light focusing device, than a focal point of one portion of the laser beam reflected by said splitting device, which is placed by said second light focusing device.

6. The optical pickup apparatus according to claim 1 , wherein if light density on said first light focusing device decreases as said collimator device is displaced close to said light receiving device, said light receiving device is disposed on a farther side, viewed from said first light focusing device, than a focal point of one portion of the laser beam reflected by said splitting device, which is placed by said second light focusing device.

7. The optical pickup apparatus according to claim 1 , wherein said second light focusing device includes at least one of a lens, a reflecting mirror, and a prism.

8. The optical pickup apparatus according to claim 7 , wherein the prism is a multiangular prism.

9. The optical pickup apparatus according to claim 7 , wherein the reflecting mirror includes at least one of a concave mirror and a convex mirror.

10. An information equipment comprising: an optical pickup apparatus comprising: an irradiating device for irradiating a laser beam to perform at least one of a recording operation and a reproducing operation on a recording medium; a first light focusing device which is disposed on an optical path of the laser beam between said irradiating device and the recording medium and which focuses the laser beam on the recording medium; a collimator device which is disposed on the optical path between said irradiating device and said first light focusing device and which can be displaced along the optical path of the laser beam; a splitting device which is disposed on the optical path between said collimator device and said first light focusing device, which reflects one portion of the laser beam, and which transmits therethrough another portion of the laser beam; a light receiving device for receiving the one portion of the laser beam reflected by said splitting device; and a second light focusing device for focusing, on said light receiving device, the one portion of the laser beam reflected by said splitting device, at least one of said light receiving device and said first light focusing device being disposed at a position at which rate of change in light density of the laser beam on said first light focusing device before and after displacement of said collimator device is substantially the same as rate of change in light density of the laser beam on said light receiving device before and after the displacement of said collimator device; and a controlling device for controlling said optical pickup apparatus to perform at least one of the recording operation and the reproducing operation on the recording medium.